In conventional motor-driven power steering systems a steering wheel is operatively connected through a steering shaft to a steering rack which is connected at its opposite ends with a pair of steerable road wheels. When the steering wheel is turned by an operator, the steerable road wheels are approximately steered in accordance with a steering force imparted to the steering wheel by the operator. On the other hand, the steering rack is operatively connected with a motor through a speed-reduction gear and an electromagnetic clutch. The driving force of the motor is transmitted through the speed-reduction gear and the electromagnetic clutch to the steering rack so as to assist the operator-induced steering motion of the steering wheel. The motor is electrically connected with a battery through a control unit and a key or ignition switch so that it is energized by the battery under the control of the control unit. The control unit is input with control signals from a steering torque sensor and a vehicle-speed sensor so as to appropriately control the operations of the motor and the electromagnetic clutch on the basis of the steering torque and the vehicle speed measured.
With the above-described conventional motor-driven power steering system, however, there is a problem that if the electromagnetic clutch is kept connected during the time when the motor fails or is mechanically restricted, the steering wheel also become arrested by the failed motor and cannot be moved or turned by the operator. To avoid this, it was proposed that the electromagnetic clutch is disconnected upon such a motor failure, for example when the steering torque sensor senses an extraordinarily large steering torque required for steering.
In this case, however, there arises another problem that the timing of disconnecting the electromagnetic clutch is unavoidably delayed, that is to say, it is difficult to disconnect the clutch immediately upon occurrence of such a motor failure since the clutch is kept connected until after the steering torque sensor senses an extraordinarily high steering torque. Thus, this solution is not so good for practical use. Further, it is necessary to make discrimination between an extraordinarily high torque required for steering resulting from mechanical restriction of the motor and that resulting from abutment of the steering wheel against a limit stop occurring when the steering wheel is fully turned to a righthand or lefthand limit position. To this end, a steering angle sensor is provided for sensing the steering angle of the front or steerable road wheels but even in this case, it is still difficult to precisely make such discrimination if an extraordinarily high steering torque is required when the steering wheel is located intermediate the righthand and lefthand limit positions. This will be caused, for example, when the front road wheels of the vehicle run onto a bump, a rock or a step on the road. In this case, it is erroneously judged that the high steering torque required, though due to large load on the front road wheels, would be caused by mechanical restriction resulting from a motor failure so that a failsafe mechanism is actuated to disconnect the electromagnetic clutch, thereby interrupting the assisting force from the motor.